The present invention relates to a metering blade for use in an electrophotographic apparatus, such as a photocopier or an LED or laser printer.
In electrophotography, dry toner is electrostatically transferred from a developing roller to a photoreceptor, and then to a printing substrate, generally paper or the like to provide a printed image on the substrate. It is vitally important that the desired quantity of toner be transferred to the photoreceptor during the printing process. A critical element in accomplishing this goal is applying a very uniform controlled layer of toner on the developing roller. A metering blade is commonly used as a tool to remove excess toner from the developing roller and to level the toner layer which electrostatically adheres to the roller.
Currently used metering blades are made from thin flexible metal which is affixed to the printer housing in a position which causes the blade to be slightly bent when installed against the surface of the developing roller, storing energy in the blade. The stored energy in the bent blade applies a small pressure to the nip which is formed between the metering blade and the developing roller. Because the blade is very thin and flexible, small deviations in the position of the blade relative to the developing roller have only a very small effect on the pressure in the nip. The flexibility of the metering blade also mitigates the need for precise parallelism between the nip and the axis of the developing roller. The pressure in the nip and its uniformity are controlled by design along with other factors to produce the proper toner layer thickness on the roller.
It is desirable to have the metering blade lay against the developing roller in a manner such that the end of the blade close to the nip is "downstream" from the nip, i.e. past the nip in the direction of rotation of the developing roller. The blade/roller configuration is commonly referred to as the "wiping position".
However, because toner is generally moved to the area of the developing roller by up to two rotating rollers, there is no room to put the metering blade in this preferred position. The metering blade must be applied in a direction at which a free end of the blade opposes the motion of the developing roller. This undesirable configuration is commonly referred to as the "doctoring" position. This configuration does not allow precise control of nip forces, and thus, cannot precisely meter the toner on the developing roller.
The problem of the metering blade facing the wrong direction (the "doctoring" position) is partially compensated for by putting a generally orthogonal bend in the metering blade near its free end, such as the metering blade disclosed in U.S. Pat. No. 5,587,551. This bend eliminates the sharp edge that could dig into and jam the developing roller. However, the flexibility of the thin blade in the vicinity of the bend and any lack of flatness that could have been mitigated by the flexibility is virtually eliminated by the bend. Also, end effects exist at both longitudinal ends of the blade and stress differentials exist in the area of the bend that contribute to a lack of flatness in the blade. This lack of flatness leads to a variation in the thickness of the toner layer along the length of the developing roller. This, in turn, can lead to process direction streaks in the printed image, reducing print quality. Secondary machining and polishing of the blade in the nip area and increased cylindricity tolerance requirements on the developing roller can improve uniformity, but these are very expensive hit-or-miss solutions.
It would be beneficial to provide a more compliant metering surface that is less subject to rigorous flatness requirements, to provide a flatter metering surface by reducing stress differentials at edges of the bend, and to provide a "wiper" blade type nip from a blade that is forced by geometrical considerations to be in a "doctor" blade configuration. The present invention satisfies these needs.